Optical couplers are devices which contain at least one optical emitter which is optically coupled to an optical detector through some sort of electrically insulating medium. This arrangement permits the passage of information from one electrical circuit which contains the emitter to another electrical circuit which contains the detector. A high degree of electrical isolation is maintained between the two circuits. Because information is passed optically across an insulating gap, the transfer is one way. That is, the detector cannot effect the input circuit. This feature is important because, for example, the emitter may be driven by a low voltage circuit using a microprocessor or logic gates, while the output photodetector may be part of a high voltage DC or AC load circuit. The optical isolation also serves to prevent damage to the input circuit caused by the relatively hostile output circuit.
Applications of this sort in which a low voltage circuit is coupled to a high voltage circuit are known as "interface" applications. Interface applications have stringent requirements for isolation voltage and minimum leakage current. As a result, optocouplers may have closely defined minimum physical dimensions for spacing between low voltage and high voltage sections. These requirements are intended to guarantee safety both for equipment and for humans, and as a result the requirements include dimensions for both electrical creepage and physical clearance.
A common optocoupler package format is the dual-in-line or DIP package. This package is widely used to house integrated circuits and was readily adapted to house optocouplers. Various versions of optocoupler DIP packages having 4, 6, 8 or 16 pins are commonly manufactured. However, the DIP package has the disadvantage that the spacing between optical emitter and optical detector is inherently limited by the size of the package. Accordingly, the internal dimensions available for electrical creepage and physical clearance are inherently limited.
According to the prior art, low cost optocoupler packages require that the device has its body molded after the process of mounting the semiconductor die and electrical connections are made. This process requires a batch processing system utilizing individual leadframes rather than a form of continuous processing. The optical path must be defined by a separate optically transmissive medium which is molded around the active devices and is then overmolded with resin or the like. This overmolded package provides physical protection, shields external light, and may also provide an integral reflective surface to aid in light transmission. The optically transmissive medium largely determines the optical performance of the assembled optocoupler. Voltage isolation thus depends on the dielectric characteristics of the optically transmissive medium and the method by which the insulation material (the optically transmissive medium) is placed within the optocoupler package. Typically a surface activation process is used to form a molecular bond between the optically transmissive medium and the surrounding mold compound in order to eliminate any path for high voltage creep. Variations in composition and amount of material applied in each unit also causes a large variation in the efficiency of the optocoupler from unit to unit.
Consequently, the need exists for a method to reduce the wide range of optical performance in variation and isolation voltage within an optocoupler while providing a low cost and easily manufactured package. It would be desirable to reduce assembly costs by eliminating expensive mold presses and surface activation equipment. Additionally, it would be desirable to provide packages suited for assembly in reeled form rather than batch lead form, and to use premolded packages for assembly.